The present invention relates to improvement of an electric power steering apparatus for drive-controlling a steering force assistant motor by setting a target value of a motor current for driving the motor for assisting steering force on the basis of steering torque.
The electric power steering apparatus drive-controls a motor for assisting steering force on the basis of a target current value determined by a steering torque. Conventional steering torque--target current characteristics are represented by a broken line which is point-symmetric with respect to the point wherein the steering torque is zero, as shown in FIG. 1. The inside part of a broken point in which the absolute value of the steering torque of the broken line is small is a low assist region in which the target current is approximately zero. The outside part of the broken point in which the absolute value of the steering torque is large is a region to assist the steering force by enlarging the target current. The region of the outside of this broken point is contrived to assist the steering force according to the running condition such that when a vehicle speed is low a gain of the target current value is enlarged to make the steering assistant force becomes large, and when the vehicle speed is high the gain is reduced to make the steering assistant force small.
However, when only the gain of the target current value is varied in accordance with the vehicle speed as described above, because of the fixation of the low assist region setting the target current to be approximately zero, freedom of the steering torque--target current characteristics is low. Accordingly, it was not possible to obtain optimum steering assistant force applicable to the vehicle speed such that, in case of the low speed, the width of the low assist region is small to give light steering feeling, and in case of the high speed, the width of the low assist region is large to give heavy (stabilized) steering feeling.